Such an earpiece end piece has become known from Fed. Ger. Pub. Sp. No. 25 48 552. The material used there has--especially in its lower hardness range--a softness comparable with the skin. This is good for the skin. On the other hand, however, a pair of spectacles--especially if it has correcting lenses--is an optical instrument which must be held very precisely at that position to which it was fitted by the optician. This signifies that the tang in this kind of spectacle earpiece must be metallic.
Now the problem consists in securing something very soft, of high quality, to something very hard, without the very soft part or the very hard part sacrificing its properties.